Electrical power systems operated by electrical utility firms and the like typically include a large number of transformers, capacitor banks, reactors, motors, generators and other major pieces of electrical equipment often interconnected with heavy duty cabling and switching devices for connecting and disconnecting the equipment to the network. Protective devices, including, but not limited to, fuses, circuit breakers, limiters, arrestors, and protective relay devices can be connected to the major pieces of equipment and are designed to open and close circuitry in the power system when fault conditions occur to protect the system from damage.
Electrical power is transmitted from substations through cables, which interconnect other cables and electrical apparatus in a power distribution network. Electrical components such as power distribution capacitors and transformers are interconnected in the network via high voltage cables, and a variety of switchgear is used to connect and disconnect power connections to the components and associated circuitry. Power switches have been used for many years to connect and disconnect power sources to loads.
Transformers are used extensively in the transmission and distribution of electrical power, at both the generating end and the customer's end of the power distribution system. Such transformers include, for example, distribution transformers that convert high-voltage electricity to lower voltage levels acceptable for use for commercial and residential customers. These include network transformers that supply power to grid-type or radial secondary distribution systems in areas of high load density. These areas of high load density include, for example, underground, metropolitan vault applications, government, commercial, institutional and industrial facilities, and office towers and skyscrapers.
Transformers are generally configured to include a core and electrical conductors that are wound around the core so as to form at least two windings (or coils). These windings or coils are typically installed concentrically around a common core of magnetically suitable material such as iron and iron alloys and are electrically insulated from each other. The primary winding or coil receives energy from an alternating current (AC) source. The secondary winding receives energy by mutual inductance from the primary winding and delivers that energy to a load that is connected to the secondary winding. The core provides a circuit for the magnetic flux created by the alternating current flowing in the primary winding and includes the current flow in the secondary winding. The core and windings are typically retained within an enclosure or tank for safety and to protect the core and coil assembly from damage. The tank also provides a clean environment, free of moisture. The tank is typically filled with an insulating fluid that provides electrical insulation function, while also serving to conduct heat from the core and coil assembly to the tank surface or cooling panels. Connections between the feeder cables and transformer core are made through under-oil bushings.
Network transformers receive power at a higher distribution voltage and provide electric power at a lower voltage to a secondary network and can include multiple switching devices. One switching device is located on the primary side (incoming power feed). This is typically a high voltage, between 13,000 volts (13 kV) and 35,000 volts (35 kV). Another switching device is on the load (customer) side and is most often designed for three-phase 120/208 volts or 277/480 volt service. The switch on the secondary side is identified as a network protector. This is a ‘smart’ switch that includes network condition assessment (i.e., load support required, network dead, network with low impedance ground path, etc.) plus sensing that there is or is not a primary voltage source applied to the transformer. This network protector also includes fuses for a secondary level of protection for the transformer.
Power is fed to the transformer at a high voltage level, through a plurality of high voltage cables. This is referred to as the ‘primary’ power and the cables are often referred to as ‘feeder’ cables. Converted power then exits the transformer through low voltage cables, which are connected to a network protector, comprising a switch on the low voltage side of the transformer.
The feeder cables and low voltage cables include electrical connection to a primary switch that allows the transformer to be disconnected and/or grounded for maintenance and testing operations in addition to network or feeder maintenance and repair activities. This ‘primary’ switch is used to control, protect, and isolate the transformer as needed. This primary switch can be mounted proximate, within, or adjacent to a network transformer and comprises a device that includes a mating pair of electrical contacts for each phase, one being stationary and one being movable, that open and close the circuit. Unless located within the transformer tank, these switches are enclosed within a separate steel housing or a housing that is part of the transformer tank, but isolated by a shared wall.
The primary switch is typically a manually-operated device with under-oil non-shielded electrical contacts. The oil, also referred to as an insulating fluid when based on non-mineral hydrocarbons, may be shared or isolated from the oil or insulating fluid used for the transformer core and coil assembly. The switch has a handle that protrudes from the transformer tank or from the independent tank. The switch may have a two-position operating mechanism or a three-position operating system. The two-position switch has a ‘ground’ or ‘clear’ position that either grounds the primary power and short-circuits the transformer primary windings or removes the shorts and grounding, thus allowing primary power to flow to the transformer (‘clear’). The three-position switch includes ‘open’, ‘closed’, and ‘ground’ positions. These positions connect the primary power supply to ground (‘ground’), isolate the primary power from the transformer (‘open’), or connect the primary power to the transformer (‘closed’). The ‘ground’ position is used for safety when the primary supply is disconnected. This safety ground prevents an accident if the primary supply is inadvertently switched on from a remote location (such as a substation).
The primary switch can be included directly in the main tank of the transformer or in an attached, smaller tank with its own oil, where the oil inside the switch is completely isolated from the oil inside the transformer tank. The location of the switch within a separate tank developed from an older design, in which the primary cables were insulated with oil and paper. These primary cables enter the tank for connection to the switch, through which contact is made with the core and coil assembly.
FIG. 1 depicts a network transformer of the prior art that comprises separate compartments for the primary cable connections and primary switch. As can be seen from FIG. 1, the unit is much more bulky than desired, which can be a major problem where space inside the transformer vault is limited.
Mounting the primary switch within the transformer tank reduces the overall footprint of the transformer. When the primary switch is mounted within the transformer tank and under the oil in the tank, the non-shielded switch contacts are exposed to the oil within the transformer tank. The primary cable connections are typically positioned on the upper surface or wall of the tank and connections are made with separable molded rubber connectors or with bolted connections within molded rubber housings. The primary switch handle is mounted on the upper surface of the tank or wall and the movement between switch positions is through a horizontal or vertical arc. The switch handle typically includes provision for a padlock to protect against inadvertent or unintended switch operation, and the lock key may be held by a supervisor, for example. Primary switches mounted in a separate housing are typically manually-operated through a vertical arc.
A failure of one or more of the major pieces of electrical equipment may require costly and time consuming delays in restoring power to customers. Failure of one or more of the major pieces of equipment may also present hazardous conditions to nearby persons and equipment. This is especially true for equipment and switchgear including components immersed in liquid dielectric fluid (oil) within a closed tank.
The primary switch is a known point of concern and risk when operating network transformers. If the switch fails, breaks, or is operated incorrectly, the risk of an under-oil arcing event is significant. Due to the high energy density, the under-oil arcing generates a large volume of oil vapor and combustible gases that can rupture a transformer tank, leading to an explosion and subsequent fire.
There are also a number of risks associated with manual primary switch operation. Firstly, the operator may not move the switch in a smooth, continuous manner, which can lead to incorrect seating of the contacts or arcing between almost-closed or almost-open contacts. In addition, the switch can become broken if the operation exceeds a stop limit or if the contacts become welded together due to heating from slow operation, for example. The switch may also be operated when the conditions are not appropriate. Finally, the operator may force operation of a damaged switch, especially since the switch contacts are not visible to the operator. These switches are normally secured with a padlock. It is during periods where the switch is unlocked and manual operation is required that increased risks arise.
To reduce risks associated with primary switch operation, some utility companies have resorted to remote switching through rope and pulley systems, for example, to allow operation of the primary switch handle from a location remote from the vault. However, this is a crude system that lacks tactile feedback and requires maintenance. The practice is not widespread, even though the risks from switch operation within a confined vault are known and significant.
More recently, some utility companies have adopted a separate, remotely-operable primary switch for their network transformers, which can include vacuum interrupters housed within a separate cabinet that is independent of the transformer tank. The separate cabinet is typically wall-mounted inside the transformer vault. These switches require re-routing of the shielded medium voltage primary feeder cables directly to the switch, with additional shielded medium voltage cables connected between the switch and transformer. However, the primary cables may not have sufficient length for addition of this type of switch and the available vault space is typically very limited. Preparation of medium voltage cable terminations is time-consuming and requires significant skill. The additional cables that run between the external switch and transformer consume available space and may impede access and escape paths in crowded vaults. Because these are rated for medium voltage applications, they have a large diameter and a large minimum allowable bending radius. These separate disconnect switches include a motor-operator or electromechanical operator.
When the primary switch is included within the transformer tank and submerged under the oil within the tank, the switch chamber is not added as an appendage to the main tank, but instead the switch contacts are bare and exposed to the oil within the transformer tank. Thus, a failure in this area can lead to a major electrical ‘event’ within the transformer tank if arcing develops and is sustained, which can cause damage to the switch, along with the core and coil transformer components. Furthermore, in some instances, failure can also cause the tank itself to rupture, potentially leading to fires and explosions.
Other configurations use a switch that is mounted on the wall of the vault, outside of the transformer tank, to fully isolate the switching function. This is a growing trend and a reaction to past primary switch ‘events’ and related concerns regarding an oil-filled switch in a confined vault where rapid egress is impossible if there is an ‘event’. In some cases, these external switches are also being added to allow remote operation, consistent with “smart grid” technology. The term “remote operation” generally means that the switch can be operated from outside of the vault, which can be as simple as a local control scheme for operation or as complex as a centralized system control communications network. The goal is to isolate personnel from the switching operation, including under-oil switches that are installed within the transformer.
However, the use of a separate primary switch adds complexity to the transformer system, with the need to run three feeder cables to the primary switch and then three more feeder cables from the switch to the transformer. Grounding connections are also required for the switch and transformer. Because of the high voltage requirements, complicated end fittings or terminations must be carefully installed for proper electric stress control.
A wall-mount switch encroaches on the valuable space in the vault, since most vaults are designed to contain the transformer, while allowing limited personnel access for maintenance and repairs, with limited provision for additional equipment. Thus, the additional cables greatly complicates the vault layout, further challenged by the limited bend radius of the feeder cables. In many cases, it is also not possible to add an external switch due to the size and layout of the vault. Finally, an external switch also adds to the inspection and maintenance burden. Vaults cannot be expanded without significant and costly encroachment on civil infrastructure.
FIG. 2 shows a top-down view of a typical vault. As can be seen from FIG. 2, the transformer is mounted against one wall and there is no available space at the primary connection end for a separate primary switch that would not significantly block personnel access or escape routes.
FIG. 3 depicts an example of a separate primary compartment mounted to a network transformer. FIG. 4 shows some of the internal switch components that correspond to this design. However, it is not believed that a separate switch compartment could be made fault-tolerant as well as the main tank, because the stiffening effect of the separate primary switch compartment would compromise the design basis of a fault-tolerant main tank, for example.
Thus, it can be seen that there remains a need in the art for an improved primary switch that can be used for controlling, protecting, and isolating transformers and other electrical apparatuses in a power distribution network and that overcomes the deficiencies of the prior art.
Vacuum interrupters were developed in the late 1960s for power switching applications and have been used in various switches, circuit breakers and other electrical power devices, including, for example, tap changers, reclosers, and as loadbreak switches.
Tap changers' are devices that are used for the momentary interruption of voltage in a power transformer between incremental changes from one tap to the next. Tap changers are typically used on the high voltage tap winding of medium voltage transformers and are not used for isolating the incoming electrical supply of the transformer.
‘Reclosers’ are switching devices that are used for power restoration and represent a specialized switch that is used to restore power to overhead or underground radial lines following an outage that may be caused by line contact with tree branches or wildlife, or a lightning strike, for example. Thus, the recloser is used to isolate power to a line when there is a fault condition, and then attempt to reconnect a fixed number of times to automatically restore power. The recloser is limited to switching operations. Reclosers do not contain any transformative features.
U.S. Pat. No. 8,284,002 to Heller et al., the subject matter of which is herein incorporated by reference in its entirety, describes a current interrupter switch for power distribution systems in which the switch is configured to fit though existing vault access holes (which are typically about 30 inches in diameter). This current interrupter switch is used as a ‘drop-in’ replacement for lower voltage oil switches and is mounted on a wall of the vault. This switch element is manually pushed in or pulled out between two electrically conductive terminals, one of which is connected to a common bus and the other of which is connected to the underground circuit. When inserted between the terminals, the switch electrically couples the terminals, completing the circuit and energizing the underground circuit. When manually pulled from the terminals, the switch breaks load current, ‘opens’ the circuit, and de-energizes the underground circuit.
U.S. Pat. No. 9,136,077 to Hu et al., the subject matter of which is herein incorporated by reference in its entirety, describes the use of three-phase, multi-way submersible loadbreak vacuum interrupter switchgear designed to replace oil-insulated and SF6 gas-insulated switchgear used in three-phase power distribution systems. The switchgear comprises a combination of electrical disconnect switches, fuses, or circuit breakers, and is used to control, protect or isolate electrical equipment for the distribution of reliable electricity within a power system. The switchgear is used to both de-energize the equipment to allow work to be conducted and to clear faults and to distribute power to different areas within the system. However, there is no indication that this type of switch assembly could be incorporated within a transformer tank for electrical connections with feeder cables and low voltage cables to isolate the transformer and allow the transformer to be disconnected from a power grid or network.
While various switch configurations have been suggested and are described in the prior art, some of which are noted above, there remains a need in the art for an improved intra-tank primary switch that overcomes the noted deficiencies of the prior art. In addition, there also remains a need in the art for an improved transformer tank design that incorporates a primary switch therein to isolate the transformer and allow the transformer to be disconnected from a power grid or network and that overcomes the deficiencies of the prior art.
Furthermore, in the event of natural disasters, such as an earthquake, utility providers may need to discontinue service to various consumers of the utility's service, as described, for example, in U.S. Pat. Pub. No. 2012/0274440 to Meadows et al., the subject matter of which is herein incorporated by reference in its entirety, because continuing to provide the utility service to a damaged or burning structure can further exacerbate the risks to those in the facility as well as emergency responders. Generally, disconnecting the utility service requires that electrical power service be disconnected at the damaged facility. In other instances, the utility may shut off large sections of its distribution system if the damage is widespread. However, doing so may interrupt utility service to areas that are not affected or to areas where the electric utilities are needed to aid with rescue and repair efforts.
Electrical utility providers may also desire to electronically communicate with key control and measurement equipment for numerous purposes including scheduling disconnection or connection of utility services to the metered loads, load shedding and load control, automatic distribution and smart-grid applications, outage reporting, and possibly for providing additional services such as Internet, video, and audio, etc. In many of these instances, in order to perform these functions, the equipment must be configured to communicate with one or more computing devices through a communications network, which can be wired, wireless or a combination of wired and wireless, as known to one of ordinary skill in the art.
In many instances, switching equipment may be designed with an electromechanical operator that can be actuated remotely to perform functions such as disconnection or connection of utility services to the metered loads, load shedding and load control, and the like. These remote switches, as well as switches in the utility's distribution system, can be used to isolate facilities that may have been damaged by seismic activity or other cause. Thus, it would be desirable for a primary switch to have remote capability such that the primary switch may be remotely operated in the event of a natural disaster or other contingency.
Furthermore, while it is known that a damaged facility can be turned off or disconnected from the distribution system, it would also be desirable for a facility to be capable of withstanding damage during a natural disaster such as a seismic event and for the transformer to be capable of continuing to provide services during such an event. This is especially critical in installations such as hospitals where a temporary loss of power can lead to disastrous results. In addition, in earthquake-prone areas it is also desirable that a facility be capable of withstanding seismic events. Thus, it would be desirable to provide a network transformer that is capable of withstanding a seismic event without damage to the transformer and without rupture or fire in the transformer that can lead to outages and loss of life.